In extruders or feed devices for thermo-plastics materials, elongated screws are rotatable in the bore of a heated extrusion barrel to mix, plasticize, and advance the plastics material from a feeder discharging solid particulate plastics material, generally in the form of pellets, to an outlet orifice for the liquid melt to be fed into the mold cavity of a molding machine. The functions of mixing, plasticizing, and conveying the plastics material are controlled by variations in the pitch, thickness and lead of the screw vanes and by the root diameters of the shaft portion of the screw. To insure complete melting of small globules of plastics material and thorough mixing of the material before it is discharged, the screws of the known extruders have had to be very long with an L/D ratio up to 30:1.